The anxiolytic activity of buspirone and structurally related compounds has been attributed to their selective activity at a serotonin (5-hydroxytyptamine; 5-HT) subtype receptor designated the 5-HT.sub.1A receptor. U.S. Pat. No. 4,202,898 discloses the treatment of anxiety and depression with a group of substituted phenylpiperazine derivatives. Among the compounds disclosed as useful for that purpose is 2,2-dimethylpropanoic acid (3-trifluoromethylthio phenyl-piperazino)ethyl ester (column 3, compound 21). The therapeutic value of 5-HT.sub.1A receptor agonists in treating multi-CNS disorders has recently been extended to compound structures such as umespirone which has high affinity for both the 5-HT.sub.1A and D.sub.2 receptor binding sites (E.P. DE No. 3529872, 87/9).